Elementary-Particle Physics: Revealing the Secrets of Energy and Matter (1998)

The field of elementary-particle physics has made dramatic progress over the past 25 years in understanding the fundamental structure of matter. Recent discoveries and technological advances are enabling high-energy physicists to address such compelling scientific issues as why elementary particles have mass, the excess of matter over antimatter in our universe, and the fundamental nature of the breaking of electroweak symmetry.

The United States is currently playing a crucial leadership role in the worldwide pursuit of the answers to these and other fundamental questions of particle physics. The accelerators and detectors at Fermilab, Stanford Linear Accelerator Center, Brookhaven National Laboratory, and Cornell are providing physicists from the United States and abroad with access to many experimental frontiers. In particular, the United States is home to the highest-energy accelerator in the world: the Tevatron at Fermilab. Furthermore, upgrades nearing completion at these facilities will ensure that the United States continues to be among the leaders in high-energy physics for well into the first decade of the next century. The elementary-particle physics program in the United States is complemented by unique facilities abroad that offer opportunities for U.S. scientists to do their research. In addition, an important part of the U.S. particle physics research program is performed without accelerators.

Leadership in elementary-particle physics research is dependent upon direct involvement in accelerator facilities operating at the high-energy frontier. In 2005, the energy frontier will move from the United States to Europe where the

Large Hadron Collider (LHC), with an energy seven times greater than the energy of the Tevatron, is now under construction. The LHC holds the promise of being a superb instrument of discovery, and there is every expectation that it will uncover important new phenomena. Maintenance of a forefront U.S. program in elementary-particle physics requires direct involvement in construction and utilization of this unique facility. The U.S. high-energy physics community is poised to play a leading role in both the construction of the LHC and its use in uncovering the physics of electroweak symmetry breaking. Although this physics is something completely new, strong theoretical arguments say that it must show up at the LHC.

The field of high-energy physics must now start to look beyond the horizon of the current program. As advised in Chapter 7, in order to explore the physics issues that are expected to remain open after the LHC, new accelerators and colliders will be needed. Given the long time scales for the design and construction of such facilities, it is essential to begin the planning process now. The ever-increasing size and cost of these facilities will require full international participation in all stages of their design, construction, and operation.

Maintenance of a leadership position in elementary-particle physics beyond the LHC era requires developing accelerator technologies to push the energy frontier ever higher, and it involves helping to build international consensus both on what technologies should be chosen for the next collider and on where the collider should be sited. Although the committee believes that it is highly desirable to have a forefront facility located within the United States, it is crucial that we maintain a technological base sufficient to allow full U.S. participation in all aspects of the design, construction, and operation of any such facility, independent of its ultimate location.

Over the past two decades, to address the important physics questions, very large experimental facilities have become the major instruments of the field. Accordingly, there have been changes in the way the field operates, particularly with respect to university groups. U.S. universities are recognized worldwide as being at the forefront of graduate education, and universities are the center of the process to educate future physicists. University groups, comprising approximately three-quarters of the experimental particle physics community, have always played a critical role in high-energy physics research. To understand how the challenging environment has affected university groups, the National Science Foundation (NSF) and the Department of Energy (DOE) have recently undertaken a comprehensive study of the situation. The committee applauds both agencies for taking this initiative.

In parallel with participation in large forefront facilities, it is essential to maintain an ability to support well-targeted areas of investigation, not necessarily at the energy frontier or accelerator-based. Much of our knowledge of the field comes from experiments of great precision or great sensitivity at lower energies that take advantage of new detector or accelerator technologies. Within

a reasonable budget, selected areas of investigation that show promise of having significant scientific impact must be supported.

It is now a fact that the time scale for construction and utilization of the primary research tools in experimental high-energy physics is well beyond a decade. This is much longer than could have been foreseen when the field began and when its management structure was set in place, and different mechanisms may be needed for its funding. Predictable funding is necessary for the effective management of research projects representing large and costly multiyear commitments. This is particularly so for international collaborations that the U.S. high-energy physics community may either join or host. In this regard, it should be pointed out that the earlier practice of full authorization at the start of major scientific projects led to efficient planning and construction.

The committee has developed its recommendations with two goals: (1) to exploit the great opportunities for discovery that lie ahead and (2) to maintain U.S. leadership in the field of elementary-particle physics. These goals require a diverse but focused program.

We are poised on the threshold of a new energy frontier, where discoveries are certain to be made, and new phenomena are likely to be revealed. This is the TeV (10¹² electron volts) mass scale, where both well-established theory and revolutionary ideas predict new physics. First, the remarkable success of the Standard Model ensures that the secret of electroweak symmetry breaking will be revealed at this scale. Second, the exciting idea of supersymmetry, which offers the hope of great insights into unification of all the forces of nature, predicts that a rich array of new particles can be produced. Finally, we will obtain the first glimpse of physics well above the typical mass scale of the Standard Model. In the past, when such a large step has been taken, dramatic experimental surprises have occurred. One might expect that similar revolutionary discoveries will be made at the TeV mass scale.

The committee therefore believes that the highest priority is full involvement in TeV mass scale physics at large facilities uniquely suited to this purpose. This involvement includes exploiting the Fermilab Collider (presently the highest-energy facility extant); strong participation in construction of and research at the Large Hadron Collider (LHC) being built in Europe; and taking a leadership role in a future forefront international facility, possibly to be built in the United States. This path has historically provided the most fruitful avenue for uncovering new phenomena.

Other problems of great importance to the understanding of elementary particles do not require the highest energies for elucidation. One is understanding rare quark and lepton transitions. Another is the nature of CP violation—a phe-

nomenon that bears on the apparent dominance of matter over antimatter in the universe. There are additional astrophysical questions of great importance that can likely be explained by particle physics dynamics, the most important being the nature of dark matter. A number of the most important findings in the field in the past two decades have been made by experiments studying problems such as these, and facilities presently being upgraded or under construction will allow such studies to continue. The committee believes it is crucial to support a well targeted program in these areas. Given the limited resources that will be available, however, maintaining a proper balance between such efforts and those at the energy frontier will require difficult choices and keen foresight.

The committee's recommendations are therefore grouped into two classes: first, those relevant to the energy frontier, and second, one concerning important studies that are best done elsewhere. Both are essential to a balanced program.

Before presenting its recommendations concerning experimental initiatives, the committee comments on two subdisciplines of the field that are critical elements of a forefront program: non-facility-specific advanced accelerator R&D, which can lead to extension of the energy frontier, and theoretical physics, which provides the framework that organizes our observations.

Advances in elementary-particle physics have historically been tied to advances in accelerator technology. Accelerator research and development is of two general types—efforts targeted at the design and construction of specific facilities and more generic (and forward-looking) R&D targeting completely new methods of acceleration that will be required to support energy frontier facilities decades from now, should the physics demand it. This report contains specific recommendations with regard to the former. It is necessary to maintain an appropriate level of investigation in the latter area to secure the longer-term future of the field.

Theoretical work in elementary-particle physics provides the intellectual foundation that motivates and interconnects much of experimental research. The more formal areas of theoretical physics, especially string theory, hold the promise of providing a picture of the universe that accounts for an extremely broad range of observations and phenomena. The committee believes that a healthy level of activity both in formal areas and in the more phenomenological investigations that touch directly on experiments now and in the coming decade should be maintained.

At the present time, the Tevatron at Fermilab and the Large Electron-Positron collider (LEP II) in Geneva are the only machines operating at the energy frontier. In two years, LEP II will be dismantled, leaving the Tevatron alone at this frontier until completion of the LHC in the middle of the next decade. The LHC will dramatically extend the energy reach, pushing beyond the

TeV scale, where we know that the physics of electroweak symmetry breaking must appear. However, this report concludes that in the future, another collider will be required to complement or extend the range of the LHC and to explore fully the physics of the TeV scale. These considerations motivate a chronological structure for the committee's recommendations concerning the high-energy frontier.

The United States should capitalize on the potential of the Fermilab Collider Facility while it has unique capabilities for investigations of high mass scale physics.

The Tevatron collider is the highest-energy accelerator in the world today and will remain so until the LHC era. The recent discovery of the top quark at this facility demonstrates its power to explore physics that is otherwise inaccessible. Its capabilities will be considerably enhanced with the new Main Injector. Although the LHC will be the first machine to extensively explore electroweak symmetry breaking, some of the new particles associated with the TeV scale might exist within the reach of the Tevatron. In particular, the upgraded Tevatron collider facility might discover supersymmetry. This would dramatically enhance our understanding of the universe.

The committee enthusiastically endorses U.S. participation in the Large Hadron Collider project as a vital and essential component of the U.S. experimental particle physics program.

In the middle of the next decade, the LHC will supersede the Tevatron Collider as the highest-energy machine in the world. U.S. physicists, with their extensive experience at Fermilab and in the research and development toward construction and use of the Superconducting Super Collider (SSC), have established critical roles in the construction of the LHC machine and of the two largest experiments. The resources involved have been established in an agreement reached in 1997 by the Department of Energy, the National Science Foundation, and CERN (the European Laboratory for Particle Physics), the host laboratory.

The LHC will systematically explore a new energy regime, the TeV mass scale. LHC experiments will elucidate the mechanism of electroweak symmetry breaking, the central question of elementary-particle physics. The experiments will decisively test the prediction that a rich array of supersymmetric particles appears at this mass scale. If supersymmetry is indeed present at the TeV scale, the LHC will initiate the exploration of a vast new world.

The committee is convinced that participation in the enormously exciting physics promised by the LHC is essential for the vitality and continuity of U.S. particle physics. The committee also believes that U.S. participation is vital for the success of the project.

As this report emphasizes, the committee anticipates that major discoveries will be made at the LHC. These will almost certainly point toward new phenomena that physicists will want to explore using an appropriate new collider.

Three types of machines have been discussed by the physics community: electron-positron linear colliders, muon colliders, and very large hadron colliders. Each has its unique capabilities and challenges, and each is at a different stage of development. Only the linear collider is far enough along to proceed to a conceptual design, with the engineering details and cost and schedule information appropriate to this stage. The other two options are sufficiently promising that increased research efforts are called for to make more realistic preliminary designs. These steps will put the community in the position to make a decision in the future about starting a new collider construction project with the best information possible.

A collider that complements or extends the reach of the LHC will require multiyear and multinational cooperation because of the magnitude of the resources needed. If the United States is to maintain a leadership role in this enterprise, it must participate both in accelerator technology development and in international decisions on the choice of technology and the location of the next facility. Although it is highly desirable to have a forefront facility located within the United States, it is crucial that the United States maintain a technological base sufficient to allow full participation in all aspects of the design, construction, and operation of such a facility, independent of its ultimate location.

Recognizing that it is too soon to endorse construction of any new machines, the committee makes recommendations concerning the development of each.

The committee recommends support of an international effort leading toward a complete design and cost estimate of an electron linear collider that would be able ultimately to reach a center-of-mass energy of 1.5 TeV and a luminosity of 10³⁴cm⁻²s⁻¹.

An electron linear collider would contribute important measurements complementary to those from the LHC toward understanding the fundamental physics of the TeV mass scale. In the past, lepton colliders have been essential complements to hadron colliders. For example, W and Z bosons were discovered

in a hadron collider, but many of their important properties could be determined only with the electron-positron colliders at LEP and the Stanford Linear Collider (SLC). For the physics of the TeV scale, this complementarity will likely continue to be important.

Laboratories in the United States, Japan, and Europe have been engaged for many years in research and development on an electron linear collider operating with an energy of 1 TeV or more. Stanford Linear Accelerator Center (SLAC), with its unique expertise in linear collider technology and the experience gained through the construction and operation of the SLC, is playing a critical leading role in these efforts. Many of the systems required for a second-generation linear collider have been or are being demonstrated. The next natural step is a complete design report, accompanied by cost optimization studies and a complete cost estimate.

The committee encourages the U.S. linear collider community to work cooperatively with international partners on the development of a common design and possible management structures.

The effort to complete an electron linear collider design and optimized cost estimate could be finished early in the next decade. It will then be necessary for the United States, together with the international elementary-particle physics community, to consider a number of factors in deciding whether to propose construction:

• The physics case for such a collider in light of any new discoveries in the intervening years;

• The construction and operating costs of the facility, together with the commitments and plans of the nations interested in hosting or participating in a linear collider; and

• The status of development of muon and hadron colliders.

R&D targeted at developing the technologies for muon and very large hadron colliders should be vigorously pursued.

Experiments at the LHC may indicate new physics at energy scales significantly beyond those that it can decisively reach. Extension of the energy frontier beyond the reach afforded by the LHC will require the development of new technologies. A muon collider or a very large hadron collider has the potential for supporting even higher energies and luminosities in the post-LHC era. R&D efforts in both of these areas are in the early stages. Muon collider technology remains to be demonstrated, so the need is to focus on the development and validation of concepts. Present-day hadron collider technology could likely be used to construct a facility with a reach significantly beyond LHC, but the cost would be prohibitive. Hence efforts in this realm should focus on a reduction of

cost through the use of advanced technologies. Development of both muon and hadron collider technologies must be pursued in a timely fashion to determine whether they represent technologically and economically viable options for reaching energies beyond those explored with the LHC.

The committee recommends strong support for a well-targeted program to study the fundamental particle physics that can best be explored with experiments below the TeV scale.

In its first recommendation, the committee has emphasized the range of important physics questions that are addressed at the TeV scale. It is important to recognize, however, that a number of outstanding fundamental questions can best be studied using other techniques. Foremost among these are the understanding of quark and lepton flavor mixing and of particle-antiparticle asymmetry (CP violation). There are also astrophysical questions of importance to particle physics, such as the nature of dark matter.

Experiments studying rare transitions between different families of quarks or leptons are extremely sensitive to new and interesting physics. For example, the 1964 experiment discovering CP violation found new fundamental physics that we are still trying to understand. One of the major themes of experimental particle physics in the next decade will be a systematic study of the interactions that mix the families of quarks and leptons.

Experiments in this area include several categories:

• Decays of the bottom quark. The central question to answer is whether CP violation is explained within the framework of the Standard Model or whether it is due to some new physics. The Standard Model explanation makes specific predictions that can be tested with very large samples of B mesons.

• Decays of the strange quark. Although CP violation was discovered in the decays of K mesons containing the strange quark, there are still outstanding issues in the CP-violating decays of strange particles. Experiments using extremely intense kaon beams give unique information about CP violation.

• Neutrino oscillations. Many experiments now give hints that a neutrino of one family can change into one of another family. One of the most important discoveries possible in the next decade would be unambiguous confirmation of any one of these hints.

A new era of research in these areas will begin in the next few years as experiments that should decisively answer many of the long-standing questions come on-line. Key U.S. facilities—the Positron-Electron Project (PEP-II) and the Cornell Electron Storage Ring (CESR) upgrade in addition to the Main Injec-

tor-will begin operations in the next few years with greatly enhanced capabilities to address this very important physics.

It is important to operate the newly built facilities and fund their critical experiments at the level required to take advantage of the physics opportunities they present. Historically, the U.S. high-energy physics community has phased out programs to accommodate those that are more scientifically desirable, and it should continue to do so. Because of limitations in resources for the field worldwide, in the future only initiatives that have the most promise for scientific advancement should be undertaken.

The recommendations above, if adopted, should maintain U.S. leadership in the field of elementary-particle physics well into the next century. They will allow our scientists to participate in what are likely to be profound and exciting discoveries, discoveries of a nature not seen before.